Wind turbines are generally classified into horizontal axis wind turbines and vertical axis wind turbines based on the layout of the rotor and blades. In a horizontal axis wind turbine, the blades are designed such that a propelling force is generated when wind flows toward the blades and as a result, blades rotate around a horizontal axis. The blades of the horizontal axis wind turbines are generally very long. The rotor and electrical generator of the horizontal axis wind turbines need to be supported on a very tall mounting tower, which may pose disadvantages in cost for installation and maintenance. Furthermore, the rotor of a horizontal axis wind turbine must be pointed into the wind. Small horizontal axis wind turbines may be pointed into the wind by a simple wind vane, while large wind turbines generally use a wind sensor coupled with an electric motor. This requirement for aiming the turbine into an incident wind direction may add to the complexity of a horizontal axis wind turbine design.
In a vertical axis wind turbine, the blades are designed such that when wind flows toward the blades, a propelling force is generated and causes the blades to rotate around a vertical axis. The rotor shaft in a vertical axis wind turbine is vertically arranged and as a result there is no need to point the turbine into the wind. This is especially advantageous on sites where the wind direction is highly variable. In vertical axis wind turbines, the electric generator and the gear box may be placed near the ground, which improves the accessibility for maintenance. A vertical axis wind turbine can be simple to design and can have low construction costs, however it has relatively low efficiency. Furthermore, the key disadvantages of a vertical axis wind turbine may include but are not limited to a relatively low rotational speed with the consequential higher torque, an inherently lower power coefficient, and a pulsating torque generated by some blade designs.
There is a need in the art for wind turbines that may address some of the above mentioned drawbacks of both vertical axis and horizontal axis wind turbines.